To perform position location in wireless cellular networks (e.g., a cellular telephone network), several approaches perform triangulation based upon the use of timing information sent between each of several basestations and a mobile device, such as a cellular telephone. One approach, called Advanced Forward Link Trilateration (AFLT) or Enhanced Observed Time Difference (EOTD), measures at the mobile device the times of arrival of signals transmitted from each of several basestations. These times are transmitted to a Position Determination Entity (PDE) (e.g., a location server), which computes the position of the mobile device using these times of reception. The times-of-day at these basestations are coordinated such that at a particular instance of time, the times-of-day associated with multiple basestations are within a specified error bound. The accurate positions of the basestations and the times of reception are used to determining the position of the mobile device.
FIG. 1 shows an example of an AFLT system where the times of reception (TR1, TR2, and TR3) of signals from cellular basestations 101, 103, and 105 are measured at the mobile cellular telephone 111. This timing data may then be used to compute the position of the mobile device. Such computation may be done at the mobile device itself, or at a location server if the timing information so obtained by the mobile device is transmitted to the location server via a communication link. Typically, the times of receptions are communicated to a location server 115 through of one the cellular basestations (e.g., basestation 101, or 103, or 105). The location server 115 is coupled to receive data from the basestations through the mobile switching center 113. The mobile switching center 113 provides signals (e.g., voice communications) to and from the land-line Public Switched Telephone System (PSTS) so that signals may be conveyed to and from the mobile telephone to other telephones (e.g., land-line phones on the PSTS or other mobile telephones). In some cases the location server may also communicate with the mobile switching center via a cellular link. The location server may also monitor emissions from several of the basestations in an effort to determine the relative timing of these emissions.
In another approach, called Time Difference of Arrival (TDOA), the times of reception of a signal from a mobile device is measured at several basestations. FIG. 1 applies to this case if the arrows of TR1, TR2, and TR3 are reversed. This timing data may then be communicated to the location server to compute the position of the mobile device.
Yet a third method of doing position location involves the use in the mobile device of a receiver for the United States Global Positioning Satellite (GPS) system or other Satellite Positioning System (SPS), such as the Russian Glonass system and the proposed European Galileo System, or a combination of satellites and pseudolites. Pseudolites are ground-based transmitters, which broadcast a PN code (similar to a GPS signal) modulated on an L-band carrier signal, generally synchronized with SPS time. Each transmitter may be assigned a unique PN code so as to permit identification by a remote receiver. Pseudolites are useful in situations where SPS signals from an orbiting satellite might be unavailable, such as tunnels, mines, buildings or other enclosed areas. The term “satellite”, as used herein, is intended to include pseudolite or equivalents of pseudolites, and the term GPS signals, as used herein, is intended to include GPS-like signals from pseudolites or equivalents of pseudolites. Such a method using a receiver for SPS signals may be completely autonomous or may utilize the cellular network to provide assistance data or to share in the position calculation. Examples of such a method are described in U.S. Pat. No. 5,841,396; U.S. Pat. No. 5,945,944; and U.S. Pat. No. 5,812,087. As a shorthand, we call these various methods “SPS”. In practical low-cost implementations, both the mobile cellular communications receiver and the SPS receiver are integrated into the same enclosure and, may in fact share common electronic circuitry.
A combination of either the AFLT or TDOA with an SPS system is called a “hybrid” system.
In yet another variation of the above methods, the round trip delay (RTD) is found for signals that are sent from the basestation to the mobile device and then are returned. In a similar, but alternative, method the round trip delay is found for signals that are sent from the mobile device to the basestation and then returned. Each of these round-trip delays is divided by two to determine an estimate of the one-way time delay. Knowledge of the location of the basestation, plus a one-way delay constrains the location of the mobile device to a circle on the earth. Two such measurements then result in the intersection of two circles, which in turn constrains the location to two points on the earth. A third measurement (even an angle of arrival or cell sector) resolves the ambiguity.
Altitude aiding has been used in various methods for determining the position of a mobile device. Altitude aiding is typically based on a pseudomeasurement of the altitude. The knowledge of the altitude of a location of a mobile device constrains the possible positions of the mobile device to a surface of a sphere (or an ellipsoid) with its center located at the center of the earth. This knowledge may be used to reduce the number of independent measurements required to determine the position of the mobile device. Typically, an estimated altitude can be manually supplied by the operator of the mobile device, or be set to an altitude from a previous three-dimensional solution, or be set to a predetermined value, or be derived from mapping information, such as a topographical or geodetic database, maintained at a location server.
U.S. Pat. No. 6,061,018, which is hereby incorporated here by reference, describes a method where an estimated altitude is determined from the information of a cell object, which may be a cell site that has a cell site transmitter in communication with the mobile device. U.S. Pat. No. 6,061,018 also describes a method of determining the condition of the measurements of the pseudoranges from a plurality of SPS satellites by comparing an altitude calculated from the pseudorange measurements with the estimated altitude.
Sometimes a table of lower resolution altitude data is stored in memory. Typically, high-resolution mapping information, such as a topographical or geodetic database, is stored in one or more flat files (non-indexed) at a location server. For example, a global Digital Elevation Model (DEM) with a horizontal grid spacing of 30 arc seconds (approximately 1 kilometer) may be obtained from U.S. Geological Survey on a set of five CD-ROMs. A DEM file from U.S. Geological Survey (http://edcdaac.usgs.gov/) is provided as 16-bit signed integers in a simple generic binary raster format. There is limited header and sometimes trailer bytes embedded in the image data. The data are stored in row major order (all the data for row 1, followed by all the data for row 2, etc.).
Sometimes, a Digital Elevation Model (DEM) is also referred to as a Digital Terrain Model (DTM).